1. Field of the Invention
This invention relates to rubber compositions, and more particularly to a rubber composition using an insoluble sulfur for preventing the blooming of sulfur in the processing of rubber mix.
2. Related Art Statement
In general, the insoluble sulfur can suppress a blooming phenomenon that sulfur migrates toward rubber surface and precipitates thereon pending the completion of vulcanization after the kneading with rubber, so that it is widely used as a vulcanizing agent in the manufacture of radial tires. The conventional insoluble sulfur is powdery, and is usually added with a process oil for uniformly dispersing the insoluble sulfur into rubber during the kneading and for preventing the scattering of fine insoluble sulfur powder in the processing operation. However, since the insoluble sulfur added with the process oil contain a large amount of process oil on its outer surface, the fluidity is poor and it is apt to adhere the process oil to a vessel wall surface of a batcher or the like, so that the processability is considerably poor. For instance, when the insoluble sulfur added with the process oil is charged from the batcher into a Banbury mixer for kneading with rubber, a part of the insoluble sulfur remains in the batcher at a state adhered to the wall surface thereof, and consequently the amount of insoluble sulfur actually charged into the Banbury mixer is changed, resulting in the scattering of properties in the kneaded rubber. Further, the insoluble sulfur is subjected to shearing force and compressive force during the kneading to form agglomerates, which remain in rubber at an insufficient dispersed state and cause the scattering of rubber properties.
In West German Patent laid open No. 1667734, there has been proposed a method of improving the fluidity of sulfur, wherein a mixture of sulfur and petrolatum was molded into a shaped body by means of a roller compression apparatus. In this case, however, the insoluble sulfur is tightly agglomerated by the application of mechanical compression force, and the resulting agglomerates remain in rubber during the kneading without being broken to particle size of the starting sulfur powder. Therefore, this method has still a drawback of causing the scattering of rubber properties. Moreover, Japanese Patent Application Publication No. 57-10,141 proposes a method of improving the fluidity of sulfur, wherein a treating oil having a surface tension reduced by adding a surfactant is added the insoluble sulfur till the starting of granule formation to obtain insoluble sulfur-containing granules. According to the latter method, the resulting granulated insoluble sulfur is good in the fluidity and is not so hard as compared with the aforementioned compression molded body, so that it is easily broken to original particle size during the kneading with rubber to uniformly disperse into rubber. However, the granulated insoluble sulfur is poor in the stability and transforms into common sulfur to cause the blooming phenomenon, and consequently the adhesive force between rubber composition and steel cord lowers.